1. Field of the Invention
The present invention relates to a method of and an apparatus for transferring a pallet, which has been supplied with a workpiece by a pallet stocker and delivered on a pallet carriage, onto the table of a machine tool and positioning the pallet on the table.
2. Description of the Relevant Art
For machining a workpiece such as a press die blank with a machine tool, it has been customary according to one practice to place and feed the workpiece on a pallet on a pallet carriage to a position near the machine tool, and then transfer the pallet from the pallet carriage to the table of the machine tool. For example, apparatus for loading and unloading workpieces between the pallet carriage and the machine tool, and positioning the workpieces on the table of the machine tool are disclosed in Japanese laid-open patent publication No. 61-168459, Japanese patent publications Nos. 63-28739 and 63-28740, Japanese laid-open utility model publications Nos. 2-42724 and 3-19651.
For smooth transfer of a pallet between a pallet carriage and the table of a machine tool, it is desirable that the height of the pallet carriage and the height of the machine tool table be the same as each other. Japanese laid-open patent publication No. 63-171797 discloses a self-propelled vehicle having a lifting mechanism. After the self-propelled vehicle loaded with a workpiece has been moved to a machine tool, the lifting mechanism is actuated to lift the workpiece up to the table of the machine tool, and then transfer the workpiece to the table.
Generally, machines tools are portal-shaped for machining workpieces from above. To machine workpieces having a certain height, such as press dies, it is necessary to lower the workpieces as much as possible on the machine tool to permit the cutter to move freely in a wide space for cutting the workpieces efficiently. Furthermore, the workpieces are required to be positioned accurately with respect to the machine tool table and fixed reliably to the machine tool table for increased machining accuracy.
There have also been demands for a compact mechanism for loading and unloading workpieces between the carriage and the machine tool, and hence for transfer apparatus which undergo less space limitations and are inexpensive to manufacture.